1. Field of the Invention
The present invention relates to a Gauss type standard photographic lens system having a large aperture ratio.
2. Description of the Prior Art
General photographic lens systems are designed mainly for favorably correcting aberrations for photographing objects at infinite distance. In recent days, however, lens performance for photographing objects at short distance is also important as well as that for photographing object at infinite distance. It is therefore ideal for a photographic lens system that it has excellent performance for photographing objects both at infinite and short distances and that the performance changes little for photographing objects at infinite and short distances. Gauss type standard photographic lenses show some degradation in performance, though not so remarkably, for photographing object at short distance. This degradation in performance can be traced mainly to aggravation of spherical aberration, especially that of short-wavelength rays which degrades image contrast.
Though a remarkable progress has recently been made in Gauss type standard photographic lens systems, the above-mentioned defect still remains in such a type of photographic lens systems that are made of glass materials having high refractive indices. For designing Gauss type standard photographic lens systems which are capable of assuring sharp image contrast, on the other hand, problems are unavoidably posed to correct flare due to coma at intermediate angle which is inherent in Gauss type lens systems and to favorably correct chromatic aberration of spherical aberration. For solving these problems, it is obliged to use glass material having a large Abbe's number, which in turn forces to adopt glass materials having low refractive indices for positive lens components. Such a selection of glass materials will bring about a drawback that it unavoidably aggravates spherical aberration and increases entire lengths of the lens systems as a whole.